Watch out for the WWI/WWII flip: in WWII (
二战)
同盟国 = the Allies (US, UK, USSR, China), and
轴心国 (zhóuxīn guó) = the Axis (Germany, Italy, Japan). But in WWI (
一战)
同盟国 means the Central Powers (Germany, Austria-Hungary, Ottomans, Bulgaria), while their opponents are
协约国 (xiéyuē guó, Entente). Same word, opposite sides — context tells you which.